The present invention relates to active dry yeast and, more particularly, to instant-type active dry yeast and a method for its preparation.
Yeast supplied to commercial bakeries as well as to consumers is generally available in two forms, i.e., as "compressed yeast" having a moisture content of from about 67 to about 73% or as "active dry yeast" having a moisture content of less than about 10%. Active dry yeast in general has the distinct advantage of being stable over prolonged periods of storage, even at elevated temperatures, but, for most common forms of active dry yeast, generally has a relatively low leavening activity (as compared to compressed yeast) and requires rehydration prior to admixture with dough ingredients in order to develop its activity. As a consequence of the lower activity, larger quantities (on a dry matter basis) of the active dry yeast are needed to obtain leavening equivalent to that obtained using compressed yeast.
In response to these disadvantages of active dry yeast, the art has developed what is known as "instant-type active dry yeast" through use of drying processes employing mild conditions in conjunction with use of yeast strains which are fairly resistant (in the sense of not undergoing a substantial loss of leavening activity) to the drying process. Such instant-type active dry yeasts are characterized (and distinguished from "conventional" active dry yeast) by their high leavening activity in direct addition methods of use, i.e., where the yeast is not rehydrated before admixture with flour and other dough or batter ingredients.
Additional characterizing features of instant-type active dry yeasts are their relatively fine particle size and relatively low moisture content as compared to conventional active dry yeast. The fine particle size contributes to the high direct addition leavening activity of the yeast by promoting its uniform dispersibility throughout the dough or batter ingredients. With respect to moisture content, the high activity of instant-type active dry yeasts, their fine particle size and their generally high protein content have the effect of rendering the dry yeast less stable on storage than a conventional active dry yeast of the same moisture content. As a consequence, production of an instant-type active dry yeast typically will involve drying to a final moisture content lower than generally required for conventional active dry yeast so as to insure adequate storage stability. Generally, the moisture content of instant-type active dry yeast is less than about 6% by weight and most typically on the order of about 4 to 5% by weight. However, in those situations where extended storage stability is not of particular concern or is attained in some manner other than reduction in moisture content, the moisture content of an instant-type active dry yeast can range as high as about 10% by weight.
In those cases where it is desired to produce an instant-type active dry yeast having a relatively low moisture content (e.g., less than about 8%, such as in the typical 4 to 5% range), the drying of the yeast suspension initially involves only the removal of free water (down to a level of about 8% water content by weight); further reduction of the moisture content to the desired low level thereafter involves the elimination of a portion of the bound water from the yeast. This latter phase of moisture reduction can cause severe damage to the yeast cell membrane. To overcome this problem, it is known in the art to conduct the drying of the yeast suspension in the presence of particular lipophilic surfactants or wetting agents such as esters of saturated fatty acids, fatty acid esters of glycerol and fatty acid esters of propylene glycol. These materials have the effect of protecting the yeast cells when the cells are dried to a low moisture content, thereby preserving the desired high direct addition leavening activity of the yeast.
Prior art processes for the production of instant-type active dry yeast are disclosed, for example, in U.S. Pat. Nos. 3,843,800 and 4,218,420 to Langejan (drying of high protein content yeast suspension under controlled conditions of time and temperature with a drying gas), and Canadian Pat. No. 1,075,077, British Patent Specification No. 1,539,211 and U.S. Pat. Nos. 4,346,115, 4,328,250, 4,318,930 and 4,396,632, all to Clement, et al. (disclosing particular yeast strains, propagation processes and controlled drying conditions).
As a result of the fact that instant-type active dry yeasts were developed in response to the low activity of conventional active dry yeast and the need to rehydrate the conventional active dry yeast and the need to rehydrate the conventional active dry yeast in water in order to develop its activity, the "acid test", so to speak, for instant-type active dry yeast has been the leavening activity achieved in direct addition methods of use, and instant-type active dry yeasts are provided to consumers and commercial bakers with the intent that they be employed in direct admixture with flour and other dough or batter ingredients.
It has been found, however, that many consumers and bakers, either as a consequence of simply being steeped in traditional methods and/or because of constraints imposed by arrangement of particular machinery and process streams in existing commercial bakery plants, nevertheless adhere to the practice of first rehydrating the yeast in water before mixing it with flour and other dough or batter ingredients. Surprisingly, it has been found that such practice results in a substantial decrease in the leavening activity of an instant-type active dry yeast (as compared to its use in direct addition processes). Hence, a product touted as being of high activity is, in these cases, perceived by the user as being a step backward. While a yeast manufacturer can make both a conventional active dry yeast and an instant type active dry yeast so as to accommodate the needs of users respectively employing rehydration methods and direct addition methods of yeast utilization, it is of course impossible for the manufacture to control the particular method which the consumer may choose to employ in any given situation.
Accordingly, a need exists for a yeast which eliminates this problem and produces leavening results which meet users' expectations regardless of whether the yeast is utilized with direct addition or rehydration methods.